The analyses of used motor oils, especially by large-scale users of oil combustion engines, has been in practice for many years. See, Brown et al., Particle Size Independent Spectometric Determination of Wear Metals in Aircraft Lubricating Oils, 1980 Anal. Chem. 52, 2365-2370.
With the development of more efficient motors and better lubricants, some of the old-fashioned and inconvenient methods based on classical chemical analysis were abandoned. In recent years, the results of motor oil analysis began to be widely used for estimating and controlling the wear of motors and kinetic mechanisms. It is known that while the motor or the mechanism operates, it sheds microparticles of the metals of which the mechanism consists and these particles get transferred into the oil or the lubricant. By ascertaining the concentrations of individual metals in lubricant and the change in concentration over time, it is possible to estimate objectively the degree of wear and the technical state of the motor, to detect in time an imminent damage, etc., provided certain standard conditions for the analysis are maintained. See, generally, Eisenstraut et al., Spectrometric Oil Analysis, Detecting Engine Failures Before They Occur, 1984 Anal. Chem. 56, 1086-1094.
Two known methods for analyzing lubricants have proved best suited so far. See, Kauffman, et al., Quantitative Multielement Determination of Metallic Wear Species in Lubricating Oils and Hydraulic Fluid, 1982 Anal. Chem. 54, 975-979. These methods are atomic emission spectrography (AES) and atomic absorption flame spectrometry (AAS). Lately the AAS is the most preferred method. In the AAS method, the sample of oil after filtering and dilution by an appropriate solvent is burnt in a special burner while the absorption spectrum is measured by a highly sensitive apparatus.
In spite of high cost, operation complications, unfitness for current workshop conditions and other limitations (see below) the spectral tribodiagnostics is at present the most successful and widely applied method of objective triboanalysis, used especially in the USA, Canada and West Europe.
There do exist, however, other methods of technical diagnosis like the vibration and acoustic method and methods based on estimation of combustion products. Their main disadvantage is that they usually only indicate the critical states of the motors and are not sufficiently sensitive and exact.
One of the limitations and drawbacks of the above spectral tribodiagnostic methods is that the information they provide concerns the total concentration of the elements and not the form in which they occur, for example their oxidation states. They do not allow determination of organic components of the samples or detection of changes in lubricants. For some non-primary metals, e.g. lead, their sensitivity is limited. Their direct applicability for analysis of dispersed abraded particles larger than 10 .mu.m is also restricted or impossible. Their relatively high cost, restricted use in current workshop conditions, limited precision, bulkiness, etc. make these methods disadvantageous.